Does too much shoulder bump decrease case lifespan?

M

Machineguntony

New member
I decided to increase shoulder bump on my 308 rounds. I was trying to make them run flawlessly in my M-60 and bolt action.

I increased the shoulder bump to the point that the finished round was lower than the first step of a Lyman case gauge. Here is a pic...

https://imgur.com/a/sEuXRsB

The round plunked perfectly into the chamber of the M-60 and cycled effortlessly in a bolt action.

I use exclusively LC 308 brass. I can usually get about 5 reloads (done by me) per case before I notice broken cases.

This time, I got a broken case every 20 or so rounds, and it was only the 2nd loading done by me. All the broken cases came from the M-60. There were no broken cases in the bolt action.

Do you think the increased shoulder bump was the cause of all the broke cases?

Does too much shoulder bump weaken the brass?
 
The more you move the brass the more your stress it. Are you annealing? If not I would recommend annealing the brass every time you reload see if this will give you a couple of extra reloads. In all honesty I don't really know if to much shoulder bump decrease the case lifespan as I only full size. But the more you work the brass the harder it gets and becomes more brittle so that is the reason you are not able to reload as much. Annealing the neck and shoulder will soften the brass.
 
To much shoulder bump? You mean your sizing more than what is necessary for the chamber? Or are you simply FL sizing back to min spec? if your getting 5 reloads out of a case, what's the problem? That's about all I ever look for! Of course I load hotter than some like but getting 5 loading's from a case doesn't bother me a bit!
 
This is how the case breaks. The flattening is not relevant; that’s from people stepping on the brass. This type of break is quite common for open bolt actions bc the chamber is larger than SAAMI spec for military reasons. However, it usually doesn’t start break until around the fifth loading. EDIT: with the increased shoulder bump, these breaks started happening on the second load.

https://imgur.com/a/twYkKDm

Fortunately, I have a tool that removes the brass quite easily.
 
Last edited:
I can't see much mystery here, and I think it would be a blessing to safely get five uses from brass run through an M-60. It's like the difference between carrying a single egg home from the supermarket carefully in a padded pillow while riding in a limousine versus sticking a stamp to your egg and dropping in it a mailbox.
 
Sevens, yes, if it was 5 loadings, but in this case the breaks started happening after two loads, as I explained in the first post

Ghbucky, I’ll take a pic of it when I get home.
 
Sevens said:
I can't see much mystery here, and I think it would be a blessing to safely get five uses from brass run through an M-60. It's like the difference between carrying a single egg home from the supermarket carefully in a padded pillow while riding in a limousine versus sticking a stamp to your egg and dropping in it a mailbox.
Click to expand...

That mirrors what I was thinking.

What problems were you having with the previous sizing setup? It sounds like the M-60 has a larger than normal chamber, so I wouldn't think you'd get that many failure to feeds unless something else was wrong.
 
Ghbucky,

It's called a broken shell extractor. Look on Brownells.com.


Machineguntony,

That's a classic head separation. When a rifle cartridge fires normally, the firing pin shoves the whole cartridge forward against the headspace surface as ignition occurs and starts building pressure. There is then a delay while the pressure gets between the case neck and the bullet to expands the neck and release the bullet and while pressure overcomes the mass inertial of the bullet and starts moving it. When gas pressure builds fast enough, as it tends to do with cartridges firing at over 30,000-40,000 psi peak pressure (lower for bottleneck cases, higher for straight wall cases), the building gas pressure expands the body of the case against the chamber wall during that delay, "sticking" the case body to the chamber. By the time pressure is free to act equally and oppositely on the back end of the case and the bullet, most of the case body is good and stuck. But the walls of the case nearer the head are thicker and take higher pressure to stick to the chamber, so they aren't stuck. The rearward force resulting from pressure is stronger than the brass, so it forces the case head to the rear with the front part of the body stuck in place. The only way that can happen is for something to give somewhere, and what gives is the thinnest brass that was a little too thick to get stuck to the chamber by pressure. It stretches out as the head moves back. This location is also called the pressure ring. You can see it in a fired case by looking at where the expansion of the diameter of the case body stops as you near the head. In effect, the inside to the case thins out there, making it weaker and also working it so it gets harder. That reduces how far it can stretch before it breaks (a material property called "elongation at break"—greatest with soft brass, approaching zero with very hard brass).

When you resize a case shorter, you increase the amount of stretch that has to occur to let the head reach the breech face. In this way you shorten how many more times you can resize the case before the pressure ring thinning and work hardening cause it to fail.
 
Yes it absolutely will reduce case life and could be dangerous as well

Yes, please define "break".
Click to expand...

A picture is worth a thousand words ;) these were loaded only 3 times and were sized as the OP describes each time .

G9JFNo.jpg


Sure we could call them separated or even split but how is that not also broke ?
 
Yep, case head separation due to over sizing. Since you increased the amount of shoulder bump, the answer is obvious: reduce the amount of shoulder bump. Personally, I would use separate brass for your MG and bolt gun.

Don
 
Unclenick,

If a 30 caliber bullet needs 5 pounds of force to push it out of the case neck, when pressure in the case is up to about 65 psi, the bullet will start moving.

Is that enough pressure to expand the case neck to chamber neck diameter?
 
Your M60 may have a long chamber with as much as .008" headspace. Your bolt action should be .004" or less. Need to measure fired rounds in each, If the M60 does have a max headspace you may need to load rounds just for it and separate M60 versus bolt action, reloaded rounds. I would try for two thousandths shoulder bump for your bolt rifle and three to four on your M60. But you need to measure fired rounds and measure shoulder setback when sizing. Hornady's Headspace Gauge Set is a good tool and works with your caliper. Measure several cases from each and adjust your sizing die to push shoulders back a measured amount. I believe Hornady's Headspace Gauges are better for this than a drop in gauge? You would also need to keep fired brass separated.
 
I think M60 military chamber maximum headspace is about 1.640" from bolt face to shoulder datum .400" diameter.

http://forums.thecmp.org/showthread.php?t=146769

If the cases have a head to shoulder dimension of 1.635" (commonly referred to as case headspace because it's measured the same as that of the headspace gauge), when the round fires head clearance from case head to the bolt face will be .005" in a 1.640" headspace chamber. The case shoulder is often set back from the force of the bolt chambering it or firing pin force which increases head clearance upon firing.

Hatcher's Notebook mentions 30-06 case shoulders being set back about .006" from the forces involved to chamber the round before firing. I've measured commercial 308 Winchester cases with up to .006" shoulder setback from a 2.5 ounce firing pin pushed by a 26 pound spring force moving about 18 fps.
 
Last edited:
Machineguntony said:
I decided to increase shoulder bump on my 308 rounds. I was trying to make them run flawlessly in my M-60 and bolt action.

I increased the shoulder bump to the point that the finished round was lower than the first step of a Lyman case gauge. Here is a pic...

https://imgur.com/a/sEuXRsB

The round plunked perfectly into the chamber of the M-60 and cycled effortlessly in a bolt action.

I use exclusively LC 308 brass. I can usually get about 5 reloads (done by me) per case before I notice broken cases.

This time, I got a broken case every 20 or so rounds, and it was only the 2nd loading done by me. All the broken cases came from the M-60. There were no broken cases in the bolt action.

Do you think the increased shoulder bump was the cause of all the broke cases?

Does too much shoulder bump weaken the brass?
Click to expand...
Yep.
 
The more you work the brass, the sooner it fails. This is a constant, though how many firing/resizing cycles it lasts varies.

Bumping the shoulder back "more" means more work on the brass, so brass life decreases.

Machineguns (and many other military arms) are generally NOT reloader friendly.

All that matters to the machine gun maker (and its military users) is that each case go in the chamber, fire, extract and eject, ONCE!

If you make a change that reduces your case life from 5 down to 2 reloads before failure, I would seriously consider undoing that change.
 
You must log in or register to reply here.
Back
Top